FIG. 1 shows one example of the construction of a single crystal pulling apparatus.
A quartz crucible 3 is disposed inside a single crystal pulling vessel 2, i. e., a CZ furnace 2. Polycrystalline silicon (Si) is heated and melted inside this quartz crucible 3. When the melting stabilizes, single crystal silicon 6 is pulled up from the silicon melt 5 inside the quartz crucible 3 by a pulling mechanism 4 using the CZ method. While this silicon is pulled up, the quartz crucible 3 is caused to rotate by a rotating shaft 10. Furthermore, the pulling mechanism 4 also rotates with respect to the pulling shaft 4a. 
During the process (one batch) in which the single crystal is pulled up, various types of evaporant are generated inside the vessel 2. Accordingly, argon (Ar) gas 7 is supplied to the single crystal pulling vessel 2, and the vessel 2 is cleaned by discharging this argon gas 7 to the outside of the vessel 2 together with the evaporants, so that the evaporants are removed from the interior of the vessel 2. The supply flow rate of the argon gas 7 is set for each process within a single batch.
Furthermore, a thermal insulating plate 8 (gas distributing column) which makes the flow of the gas 7 inside the single crystal pulling vessel 2 orderly and guides the gas to the surface 5a of the melt 5, and which insulates the single crystal silicon 6 from the heat source, is disposed around the circumference of the single crystal silicon 6 above the quartz crucible 3. The distance of the gap between the lower end of the thermal insulating plate 8 and the melt surface 5a (hereafter referred to as “D0”, see FIG. 1) is appropriately set.
Oxygen is present in solid solution inside the single crystal silicon 6 that is pulled up and grown. This oxygen dissolves in the silicon melt 5 from the quartz crucible 3, and is incorporated into the single crystal silicon 6 when the single crystal silicon 6 is pulled up. The oxygen concentration in the single crystal silicon 6 has a great effect on the characteristics of the resulting element or device, and also has a great effect on the yield in the manufacturing process of the element or device.
FIG. 2 shows the relationship of the amount of oxygen that is dissolved in the melt 5 from the quartz crucible 3, the amount of oxygen that evaporates from the surface 5a of the melt 5, and the amount of oxygen that is incorporated into the single crystal silicon 6. As is shown in FIG. 2, The amount of oxygen that is incorporated into the single crystal silicon 6 is an amount that is obtained by subtracting the amount of oxygen that evaporates from the melt surface 5a from the amount of oxygen that is dissolved in the melt 5 from the quartz crucible 3. Generally, approximately 99% of the oxygen that is dissolved in the melt 5 from the quartz crucible 3 evaporates; the remaining 1% (approximately) is thought to be incorporated into the single crystal silicon 6.
Accordingly, the oxygen concentration in the single crystal silicon 6 can be controlled by controlling two quantities: i.e., the amount of oxygen that is dissolved in the melt 5 from the quartz crucible 3 and the amount of oxygen that evaporates from the melt surface 5a. 
Here, the amount of oxygen that is dissolved from the quartz crucible 3 is determined by parameters such as the rpm of the quartz crucible 3, the heating temperature of the quartz crucible 3 and the like.
Conventionally, therefore, inventions for controlling the oxygen concentration in the single crystal silicon to a desired concentration by adjusting parameters such as the rpm of the quartz crucible 3 and the like are publicly known techniques for which patent applications and the like have been filed (for example, official gazettes of Japanese Patent Application Laid-Open No. 10-167881 and Japanese Patent Application Laid-Open No. 10-167892).
Furthermore, the amount of oxygen that evaporates from the melt surface 5a is determined by parameters such as the flow rate of the argon gas 7, the pressure inside the furnace, D0 and the like.
Conventionally, therefore, inventions for controlling the oxygen concentration inside the single crystal silicon to a desired concentration by adjusting parameters such as D0 and the like are publicly known techniques for which patent applications and the like have been filed.
Inventions relating to the control of the “amount of dissolved oxygen” using the heating temperature of the quartz crucible 3 as a parameter include the inventions described below.
Specifically, in the official gazette of Japanese Patent No. 3000923, an invention is described in which upper and lower heaters 9a and 9b which allow independent adjustment of the heating applied to the quartz crucible 3 are installed around the circumference of the quartz crucible 3 along the vertical direction of the quartz crucible 3 as shown in FIG. 5, and the amount of dissolved oxygen is controlled by setting the ratio of the output of the upper heater 9a to the total output of both heaters 9 at a specified value, so that the oxygen concentration in the single crystal silicon 6 is kept to a target oxygen concentration or less.
Furthermore, an invention in which heaters are respectively installed around the circumference of a quartz crucible and in the bottom part of this quartz crucible, and the amount of dissolved oxygen is controlled by adjusting the outputs of these heaters so that the oxygen concentration in the single crystal silicon is controlled is described in the official gazette of Japanese patent No. 2681115.
However, the inventions described in these official gazettes are inventions that control “the amount of oxygen that is dissolved”, not “the amount of oxygen that evaporates”. Accordingly, the oxygen concentration range in the single crystal silicon 6 is restricted, so that this oxygen concentration cannot be freely varied over a broad range. Furthermore, there are also limits to how far the variation in the distribution of the oxygen concentration in the axial direction (direction of crystal growth) of the single crystal silicon 6 can be reduced.
Furthermore, in the case of Japanese Patent No. 3000923, no thermal insulating plate 8 is provided; accordingly, the oxygen concentration control level required in the large-diameter single crystal silicon ingots used today cannot be achieved, and in some cases, it may be impossible to pull large-diameter single crystal silicon ingots.